The most common two-dimensional display system is the cathode ray tube which is employed in such widely diverse applications as electronic instrumentation and metering equipment, television broadcast receivers, and data and word processing systems. This diversity of applications notwithstanding, employment of the cathode ray tube in such systems presents substantial problems. For example, it is difficult and expensive to drive a tube with solid-state circuitry. Extremely high voltages are required and power consumption is relatively high. The tube is very large compared to the display area and, for many applications, cannot be practically made larger than about twenty-five inches in diameter. In addition, the images which appear on a cathode ray tube are of relatively low intensity and are not as visible under bright lighting conditions.
A number of systems have been proposed as alternatives to the cathode ray tube. One such class of devices is the liquid crystal. This device will operate at relatively low voltages and with a very low level of power consumption. However, the liquid crystal, which is normally used as a reflective device, experiences only a very subtle change in its reflectivity when it is activated. Accordingly, it is difficult to discern information displayed on a liquid crystal under a variety of conditions.
Nevertheless, in spite of its drawbacks, the liquid crystal has seen application in such areas as calculators and watches, because of its advantages compared to conventional cathode ray tube displays.
Another alternative to the cathode ray tube is the light emitting diode or LED. While, dependent upon the material of which it is made, an LED will emit coherent radiation of a given wavelength, perhaps the most commonly used LED is made of gallium arsenide and emits red light when it is activated. Compared to liquid crystal displays, LED displays are quite vivid. They also retain the advantage of low-voltage drive circuitry. Because of these characteristics, they have seen exceptionally wide application in such devices as watches, calculators and are now even being used as alpha-numeric displays on memory typewriters and other business equipment systems.
However, the LED has a number of serious drawbacks. Problems include bigger expense per square inch and high power consumption of the LED as compared to the liquid crystal. Thus, a large display would necessarily require a great number of light emitting diodes and would have a correspondingly enormous level of power consumption.
Moreover, LED display systems are not practical under bright lighting conditions such as sunlight, which tends to overpower the display in the same manner that it overpowers cathode ray tube displays.
Perhaps the newest technology to be applied to the field of displays is the electrostatic display. Such a system is shown in U.S. Pat. No. 3,553,364 issued to Lee on Jan. 5, 1971. This patent illustrates a light valve which operates primarily as a light transmissive device. In one embodiment, Lee discloses a device which comprises a housing which is substantially transparent except for a pair of leaf shutters which are disposed adjacent each other in the center of the housing and secured to a common support at their bases. When activated the unsecured portion of each leaf is attracted toward the housing, causing the leaves each to extend toward an edge of the housing opposite from its support point, thus blocking the passage of light through the housing. Thus, when the housing is backlight, it appears bright in the unactivated state and dark when activated. However this device also has a number of disadvantages. For example, backlighting the device requires a relatively large amount of power. Moreover, because of the fact that it is a light emitting device, it would appear that the device would not be an effective display in environments where the ambient light is at relatively high intensity levels.
Another approach in using electrostatic technology is shown in U.S. Pat. No. 3,648,281 issued to Dahms et al on Mar. 7, 1972. Dahms discloses an electrostatic display panel which comprises a pair of planar square fixed electrodes supported at a relatively wide angle with respect to each other to form a V-shaped groove. At the bottom of the groove, an electrically conducting leaf is hingedly secured to the bottom of the groove. One of the fixed electrodes is painted black and the other is painted white. The side of the leaf facing the black electrode is painted black and the side of the leaf facing the white electrode is painted white. In operation, the leaf is attracted to one electrode or the other, thus modulating the intensity of reflected light. This device has excellent characteristics under high intensity lighting conditions because, like the liquid crystal, it is purely a reflective device. However, unlike the liquid crystal it has excellent contrast. However, it does suffer from several problems. Primarily, Dahms chooses a relatively wide angle because of what he characterizes as the advantageous viewing characteristics of a wide-angle groove structure. One would expect this because making Dahms structure with a narrow angle would hinder the display considerably due to the effect of shadows and distortion. However, the use of a wide angle structure necessitates excessively high driving voltages and rather clumsy leaf hinging structures. Moreover, the response time of a wide angle structure is relatively slow, making driving at video speeds of only limited value.
In my earlier issued U.S. Pat. No. 3,897,997, the disclosure of which is hereby incorporated by reference, I described an electrostatic display device with variable reflectivity in which the variable electrode is metalized and wraps over the fixed electrode to change its appearance. The device described there includes a fixed electrode which has an effective operating surface that is curved and has at least 120 degrees of arc in order to achieve wide effective viewing angle. The viewing angle is enhanced by reflection of the fixed electrode in the metalized variable electrode or flapper.
In my earlier issued U.S. Pat. No. 4,094,590, the disclosure of which is hereby incorporated by reference, there is disclosed a display device including a fixed electrode and a flapper. In this device, the flapper is given a bend in order to provide an advantageous mounting structure.
In my earlier U.S. Pat. No. 3,989,357, an electrostatic device useful for modulating transmitted light is described. In particular, the system there disclosed, comprises a light source which backlights the device which may be activated to prevent transmission of light through the device.